The present invention relates to load-bearing structures comprising two or more layers, preferably elongate or oblong layers, and in particular glued structural members.
Load-bearing structures occur in various shapes, sizes and connections. They are practically indispensable in construction activities, either as parts of the construction itself, or as aids for the construction work (e.g. scaffolds), or both. They are also frequently used in smaller constructions such as furniture, like for instance chairs, tables and sofas. Load-bearing structures also constitute parts of various tools such as axes and sledges, in the form of handles, but also as power-transmitting or supportive parts of machines and apparatuses. As the word implies, the primary purpose of a xe2x80x9cload-bearing structurexe2x80x9d is to carry loads, give support to other parts in a construction, and/or to transmit dynamic forces in a construction or a machine; thus other properties, such as for instance vibration or sound insulating properties, if any, may be considered to be of minor significance with regard to the present structure. Considering a floor, for instance, it is the floor-joists that constitute the load-bearing structures, whereas the flooring, the purpose of which is to provide for a surface to walk on and to place objects on, to be decorative, and to provide for insulation from cold and noise, does not constitute such a structure.
Depending on the specific use intended for the load-bearing structure it may be made of different materials, such as metal, wood or plastic, or combinations thereof. In many instances such structures are made of two or more layers. As a matter of fact, a load-bearing structure may in some cases be made stronger if split into two or more layers and then rejoined by means of some suitable adhesive, as has been shown by P. J. Gustafsson in xe2x80x9cAnalysis of generalized Volkersen-joints in terms of non-linear fracture mechanicsxe2x80x9d (Mechanical Behaviour of Adhesive Joints, 1987, pp. 323-338), hereby incorporated by reference.
Frequently, load-bearing structures of the latter kind are so-called glued structural members, especially glued-laminated timbers, extensively discussed in chapter 10 of xe2x80x9cWood Handbook: Wood as an engineering materialxe2x80x9d (U.S. Forest Laboratory, 1974, USDA Agr. Handb. 72, rev), incorporated herein by reference. The strength of such a laminated product depends, according to said xe2x80x9cWood Handbookxe2x80x9d, upon the quality of the adhesive joints. The joints may in principle be divided into two groups, ductile and brittle, although in practice most joints will be found in the transition region between those two extremes. The concept of ductile and brittle joints has been extensively discussed by P. J. Gustafsson (see supra). Ductile characteristics are often preferred to brittle, e.g. for joints subjected to stress concentrations (described e.g. in McGraw-Hill encyclopaedia of Science and Technology, 1960, Vol. 3, page 185), joints big in size, joints subjected to dynamic loads, especially impact loads, and joints subjected to forces from shrinkage or swelling of joined wooden layers making up load-bearing structures. If it is not desirablexe2x80x94or possiblexe2x80x94to modify such joint parameters as size, geometry, or properties relating to the layers making up the load-bearing structure, the only option left for regulating the brittleness/ductileness properties of the joint is to change the parameters relating to the bond line. H. Wernerson and P. J. Gustafsson in xe2x80x9cThe complete stress-slip curve of wood-adhesives in pure shearxe2x80x9d (Mechanical Behaviour of Adhesive Joints, 1987, pp. 139-150) teach that the question whether a bond line will be brittle or ductile may generally be determined by the relation between the peak shear stress and the fracture energy of the bond line: thus a low peak shear stress, in relation to the fracture energy, indicates a ductile performance, and vice versa. As a low peak shear stress for quite obvious reasons is not desirable, the fracture energy has to be increased in order to obtain a ductile joint.
H. Wernerson and P. J. Gustafsson (see supra) have performed a number of tests regarding the performance of bond lines consisting of PVAc, polyurethane and resorcinol/phenol adhesives between wooe adherents; the fracture energy of the bond lines ranges from about 0.4 to 2.4 kNm/m2. A similar test is described in P. J. Gustafsson (see supra), in which the strength of joints with bond lines of polyurethane and resorcinol/phenol adhesives is compared with the strength of solid wood; the polyurethane bond line gave the highest strength, 2.1 MPa.
Takeshi Sadoh et al have studied the bending properties of horizontally veneer-laminated beams with glue-layers of various kinds, including an elastomeric adhesive, and compared said properties with those of vertically veneer-laminated beams, considered to be equivalent to solid wood beam (xe2x80x9cVeneer laminates with elastomeric glue-layers. I. Rending properties of veneer-laminted beamsxe2x80x9d, Mokuzai Gakkaishi, 1978, 24(5), page 294-8).
Accordingly, all indications in the prior art relating to enhanced ductility of joints in load-bearing structures due to increased fracture energy of the bond lines are directed to bond lines consisting of glue, and n thing but glue.
The prior art further indicate a number of other laminated products with glue lines of elastomeric adhesives, and even laminates comprising elastomeric sheets, e.g. in U.S. Pat. No. 5,103,614, JP 1244838 and JP 61-261048; however, all of these suggestions relate to sound insulating and vibration damping properties useful in e.g. flooring material; said properties are due to a hysteresis phenomenon associated with the elastomeric materials used, which phenomenon basically consists of a transformation of the mechanical energy of the sound or vibrational energy into thermal energy.
The problem to be solved by the present invention is to provide a load-bearing structure of the kind referred to in the introduction having joints with enhanced ductility when compared to prior art load-bearing structures of similar kind and size.
This problem is solved by the present invention, which relates to a load-bearing structure of the kind referred to in the introduction and has the additional features as defined in the characterising clause of appended Claim 1. More specifically the load-bearing structure according to the present invention comprises two or more layers, at least two of which are joined by a bond line, that has a thickness of t mm, a shear strength of T N/mm2, and which comprises a sheet made of an elastic material, which has a shear modulus of G N/mm2, whereby       tT    2    G
is at least about 5 N/mm, preferably at least about 10 N/mm, and most preferably at least about 50 N/mm. If the structure comprises more than two layers they may all be joined by sheets of the present kind, but it may alternatively contain only one such sheet while the layers otherwise are joined by means of conventional bond lines.
Bond lines of the kind present in load-bearing structures of the instant invention, i.e. the sheet plus adhesive, have shown to have about twice the load-bearing capacity of bond lines according to prior art, even though the bond line according to the invention was applied to a comparably smaller joint area.
Apart from solving the stated problem, the present invention provides a number of advantages. The present invention creates opportunities for a number of combinations of sheet material, sheet thickness, and adhesives, providing for great freedom of choice with regard to joint properties. The joint may for instance be established very quickly, as a prefabricated sheet can be adhered by means of a suitable fast drying or fast curing glue or adhesive. The shear strength of the adhesive used in the inventive bond line does not have to be as high as for those used in prior art bond lines (which usually have a strength of about 10-15 MPa). Furthermore the bond line thickness, which of course is a significant joint parameter, may be chosen arbitrarily as the sheet used in the bond line can be prefabricated with any thickness, which would not be practicable if only elastomeric adhesives were to be used in a bond line according to prior art.
Any material having the properties stated in Claim 1 may of course be used in the inventive load-bearing structure, but preferably the sheet will be made of some conventional elastomeric material such as a natural or synthetic rubber, or a mixture thereof. A preferred group of rubbers is nitrile rubbers.
The shear strength of the elastic material is suitably no less than 2 MPa, and the thickness of the sheet is normally about 0.1-5 mm, preferably about 0.2-3 mm.
The layers making up the load-bearing structure may be made of any suitable material used in the art, such as for instance wood, metal, plastics, ceramics, or combinations thereof. According to one embodiment at least one of the layers of the load-bearing structure is made of wood. In another embodiment at least one of said layers is made of metal. Commonly the present structure is made mainly of wooden layers, such as for instance in glued-laminated timbers and other similar wood based load-bearing structures.
The present invention also relates to the use of a structure as described above as a load-bearing structure.
Furthermore the present invention relates to a method for producing a load-bearing structure of the present kind, i.e. as described above, by treating a sheet of natural or synthetic rubber, or a mixture thereof, having suitable shape and size with an oxidant, preferably substantially concentrated H2SO4, removing excess oxidant, and then gluing the sheet between two layers. The treatment with the oxidant should be kept for a period of time sufficiently long to oxidise the sheet to such a degree that the adhesion between the sheet and the glue is satisfying, while on the other hand it should be sufficiently short to avoid a too thick oxide coating, which could make the sheet surface brittle. Preferably the treatment with oxidant is kept for a period of about 10-25 seconds, particularly 13-20 seconds. The treated sheet preferably comprises nitrile rubber.